Davis, 62, and his wife, LaDonna Davis, 64, were visiting the sanctuary to celebrate the birthday of a 39-year-old chimpanzee, Moe, who was taken from their home in 1999 after biting off part of a woman's finger.

The couple had brought Moe a cake and were standing outside his cage when the two young male chimps, Ollie and Buddy, attacked the man.

Two other chimps, females named Susie and Bones, also escaped from the cage. They were recovered outside the sanctuary five hours later.

Susie and Bones could have played a role in the attack, primate experts said.

"We know that one of the most reliable predictors of increased male aggression is the presence of sexually receptive females," said Jeffrey French, a psychobiologist who studies primate behavior at the University of Nebraska, Omaha.

The chimps chewed off most of Davis' face, tore off his foot and attacked his limbs and genitals. Davis was transported to Loma Linda University Medical Center, where he had surgery late Thursday night.